1. Field of the Invention
The invention is in the field of corrugated cardboard fabrication.
2. Brief Description of the Background Art
The invention relates to a preheating device for locally controllable preheating of a material web comprising at least one smooth web and/or at least one corrugated web and moving in one direction, before any adhesive is applied to the material web and before the web is glued to at least one other material web to form a corrugated cardboard web. The device comprises a preheating element that is adapted to be fed with heat transfer fluid and that is in contact with the material web at a contact surface extending across the entire width of the material web.
A problem that frequently arises in practice is that the material web has "wet strips", or a moisture distribution varying across its width. These wet strips result in uneven drying of the adhesive during the gluing of the material webs, which results in warping of the corrugated board and hence poor quality corrugated board products. More intensive or less intensive heating varying across the material web width corresponding to the respective moisture content would enable any moisture differences of this kind in the web of material to be compensated before the application of adhesive or before gluing.
For example, the article "Widmer-Walty installs infra-red drying systems in their corrugator" by Michael Brunton, published in the journal "International Paperboard Industry", issue of September 1994, describes an arrangement in which, in addition to uniform contact heating by a preheating cylinder fed with steam, the web of material can be irradiated by means of an array of individually controllable infrared lamps placed across the entire width of the web of material. However, locally selective preheating by means of an infrared lamp arrangement has considerable disadvantages. For example, additional space is required for installation of the infrared radiation unit inside the preheating device and hence inside the corrugated cardboard machine. There is also a risk that the material web may ignite if the infrared radiation is too intense. As a safety measure it is therefore necessary, as described in the article, to install an additional sprinkler installation in order to extinguish any ignited material web portions. This further increases the cost of the equipment and requires increased space for the installation of the preheating device. Obviously such a preheating device is expensive both to purchase and maintain, particularly because of the safety precautions and the high power required for the infrared lamps.
In addition to uniform preheating by means of a steam-fed preheating cylinder, locally selective preheating over the width of the material web by microwave radiation or by inductive heat generation is known in the industry. Reference should be made, for example, to the text of the Paper "Temperaturkontrolle an der Wellpappenanlage" by E Bradatsch, BHS Corrugated, Germany, of the 9th Technischer Seminar der FEFCO (Apr. 22-25 1997, Nice, France).
In addition, DE 34 00 333 C2 discloses a drying device in which, after gluing, the joined corrugated web can be supplied with locally variable heat by means of spaced-apart, separately controllable plate elements for the purpose of evenly drying the adhesive.